feat: implement account management for multi-account Claude Code (gt-3133)

Adds support for managing multiple Claude Code accounts in Gas Town:

- accounts.json config parsing in mayor/ directory
- gt account list/add/default commands
- GT_ACCOUNT env var support with priority resolution
- --account flag on gt spawn and gt crew at commands
- CLAUDE_CONFIG_DIR injection into tmux sessions

Priority order: GT_ACCOUNT env var > --account flag > default from config

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/loader.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 	"time"
 )
 
@@ -417,3 +418,150 @@ func NewMayorConfig() *MayorConfig {
 		Version: CurrentMayorConfigVersion,
 	}
 }
+
+// LoadAccountsConfig loads and validates an accounts configuration file.
+func LoadAccountsConfig(path string) (*AccountsConfig, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf("%w: %s", ErrNotFound, path)
+		}
+		return nil, fmt.Errorf("reading accounts config: %w", err)
+	}
+
+	var config AccountsConfig
+	if err := json.Unmarshal(data, &config); err != nil {
+		return nil, fmt.Errorf("parsing accounts config: %w", err)
+	}
+
+	if err := validateAccountsConfig(&config); err != nil {
+		return nil, err
+	}
+
+	return &config, nil
+}
+
+// SaveAccountsConfig saves an accounts configuration to a file.
+func SaveAccountsConfig(path string, config *AccountsConfig) error {
+	if err := validateAccountsConfig(config); err != nil {
+		return err
+	}
+
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return fmt.Errorf("creating directory: %w", err)
+	}
+
+	data, err := json.MarshalIndent(config, "", "  ")
+	if err != nil {
+		return fmt.Errorf("encoding accounts config: %w", err)
+	}
+
+	if err := os.WriteFile(path, data, 0644); err != nil {
+		return fmt.Errorf("writing accounts config: %w", err)
+	}
+
+	return nil
+}
+
+// validateAccountsConfig validates an AccountsConfig.
+func validateAccountsConfig(c *AccountsConfig) error {
+	if c.Version > CurrentAccountsVersion {
+		return fmt.Errorf("%w: got %d, max supported %d", ErrInvalidVersion, c.Version, CurrentAccountsVersion)
+	}
+	if c.Accounts == nil {
+		c.Accounts = make(map[string]Account)
+	}
+	// Validate default refers to an existing account (if set and accounts exist)
+	if c.Default != "" && len(c.Accounts) > 0 {
+		if _, ok := c.Accounts[c.Default]; !ok {
+			return fmt.Errorf("%w: default account '%s' not found in accounts", ErrMissingField, c.Default)
+		}
+	}
+	// Validate each account has required fields
+	for handle, acct := range c.Accounts {
+		if acct.ConfigDir == "" {
+			return fmt.Errorf("%w: config_dir for account '%s'", ErrMissingField, handle)
+		}
+	}
+	return nil
+}
+
+// NewAccountsConfig creates a new AccountsConfig with defaults.
+func NewAccountsConfig() *AccountsConfig {
+	return &AccountsConfig{
+		Version:  CurrentAccountsVersion,
+		Accounts: make(map[string]Account),
+	}
+}
+
+// GetAccount returns an account by handle, or nil if not found.
+func (c *AccountsConfig) GetAccount(handle string) *Account {
+	if acct, ok := c.Accounts[handle]; ok {
+		return &acct
+	}
+	return nil
+}
+
+// GetDefaultAccount returns the default account, or nil if not set.
+func (c *AccountsConfig) GetDefaultAccount() *Account {
+	if c.Default == "" {
+		return nil
+	}
+	return c.GetAccount(c.Default)
+}
+
+// ResolveAccountConfigDir resolves the CLAUDE_CONFIG_DIR for account selection.
+// Priority order:
+//  1. GT_ACCOUNT environment variable
+//  2. accountFlag (from --account command flag)
+//  3. Default account from config
+//
+// Returns empty string if no account configured or resolved.
+// Returns the handle that was resolved as second value.
+func ResolveAccountConfigDir(accountsPath, accountFlag string) (configDir, handle string, err error) {
+	// Load accounts config
+	cfg, loadErr := LoadAccountsConfig(accountsPath)
+	if loadErr != nil {
+		// No accounts configured - that's OK, return empty
+		return "", "", nil
+	}
+
+	// Priority 1: GT_ACCOUNT env var
+	if envAccount := os.Getenv("GT_ACCOUNT"); envAccount != "" {
+		acct := cfg.GetAccount(envAccount)
+		if acct == nil {
+			return "", "", fmt.Errorf("GT_ACCOUNT '%s' not found in accounts config", envAccount)
+		}
+		return expandPath(acct.ConfigDir), envAccount, nil
+	}
+
+	// Priority 2: --account flag
+	if accountFlag != "" {
+		acct := cfg.GetAccount(accountFlag)
+		if acct == nil {
+			return "", "", fmt.Errorf("account '%s' not found in accounts config", accountFlag)
+		}
+		return expandPath(acct.ConfigDir), accountFlag, nil
+	}
+
+	// Priority 3: Default account
+	if cfg.Default != "" {
+		acct := cfg.GetDefaultAccount()
+		if acct != nil {
+			return expandPath(acct.ConfigDir), cfg.Default, nil
+		}
+	}
+
+	return "", "", nil
+}
+
+// expandPath expands ~ to home directory.
+func expandPath(path string) string {
+	if strings.HasPrefix(path, "~/") {
+		home, err := os.UserHomeDir()
+		if err == nil {
+			return filepath.Join(home, path[2:])
+		}
+	}
+	return path
+}